1. Field of the Invention
The present invention relates to an AC current source circuit for providing a high-frequency AC current for a capacitive load such as an ozone generator, and more specifically to an AC current source circuit for converting a DC input voltage into a DC current by a chopper circuit and a direct current (DC) reactor, and converting the DC current into an AC output current by a semiconductor current-fed inverter.
2. Description of the Related Art
FIG. 1 shows the configuration of a conventional AC current source circuit. Described below are the configuration and operation of the circuit.
In FIG. 1, a 3-phase AC voltage generated by a 3-phase AC current power source 1 is converted into a DC current by a thyristor rectifier 2 and a direct current (DC) reactor (DCL) 3. The DC current is converted into an AC current of a rectangular waveform through a current-fed inverter (thyristor inverter) 4, and provided to a capacitive load 5, which is represented by a parallel circuit of a capacitor and a resistor.
In FIG. 1, a through f are thyristors which form a thyristor rectifier 2, and 6 through 9 are thyristors which form a current-fed (current-type) inverter 4. 10 through 13 are snubber circuits each of which is obtained by connecting a resistor and a capacitor in series. The snubber circuits 10 through 13 are connected to the thyristors 6 through 9 respectively in parallel.
For example, when the current flows through the thyristor rectifier 2, DCL 3, thyristor 6, capacitive load 5, thyristor 9, and back to the thyristor rectifier 2 in the polarity inverting operation for an output current by the current-fed inverter 4, if the thyristors 7 and 8 are turned on, the thyristors 6 through 9 all indicate conduction state because the load 5 is capacitive.
Thereafter, the thyristors 6 through 9 are turned off by the charged voltage of the capacitive load 5.
As a result, the current flows through the thyristor rectifier 2, DCL 3, thyristor 8, capacitive load 5, thyristor 7, and back to the thyristor rectifier 2, thereby inverting the polarity of the output current. A similar operation is performed in the opposite polarity inverting operation.
The snubber circuits 10 through 13 are connected to the thyristors 6 through 9 respectively in parallel in order to protect each of the thyristors 6 through 9 from the surge voltage generated when the thyristors 6 through 9 turn off.
The conventional device shown in FIG. 1 includes charge/discharge type RC snubber circuits, each of which comprises a series circuit of a resistor and a capacitor, as the snubber circuits 10 through 13 for protecting semiconductor elements for an inverter, that is, the thyristors 6 through 9.
When the resistance value of the resistor is Rs and the capacitance of the capacitor is Cs in each of the snubber circuits 10 through 13, the generation loss in each of the snubber circuits 10 through 13 is equal to or larger than Cs.times.Vop.sup.2 .times.f (Vop indicates a peak value of the output load voltage, and f indicates the output frequency).
Since this loss increases when the source circuit is used for a device which requires a high frequency or a high output load voltage, a large snubber circuit is required, which makes it difficult to make the entire system small, light in weight, with high efficiency, and less expensive.
Furthermore, since a DC current power source is prepared using the thyristor rectifier 2 which is switched at a frequency of the commercial power supply according to the conventional circuit shown in FIG. 1, a low-frequency ripple voltage on a cycle 6 times as long as the frequency for commercial power supply is applied to the DCL 3. Therefore, the DCL 3 has been required to have a large capacity.